Tons of lead melted and spread when the cathedral caught fire. Recent tests found lead levels of more than 1 million micrograms per square meter in the cathedral's square.

Robin des Bois, a French environmental group, said in a statement back in April, "The cathedral has now become filled with toxic waste."

Workers will use an absorbent gel and high-pressure water jets to clean the area contaminated with lead. City officials said decontamination operations will take three to four weeks. Reconstruction of the cathedral is expected to resume on Monday.